Particle beam injection system

ABSTRACT

A beam of electrons is injected into a plasma confining magnetic field by directing the beam of particles along a trajectory which is generally parallel to the lines of force of the magnetic field with drift forces existing on the particles to cause them to drift across magnetic lines of force by either the use of an auxiliary magnetic field or by adjusting the parameters of the toroidal system when injecting into a toroidal confining magnetic field. The system is also adaptable to simultaneous injection of a plurality of beams and for injection times greater than one transit of the torus.

ilnited 8tates Patent [191 Benford et a1.

[11] 3,831,181 [4 1 Aug. 20, 1974 PARTICLE BEAM INJECTION SYSTEM [73] Assignee: Physics International Company, San

Leandro, Calif.

[22] Filed: Mar. 5, 1973 [21] Appl. No.: 338,089

[52] US. Cl 328/230, 313/79, 315/27 R [51] Int. Cl. Hillj 29/76 [58] Field of Search 315/27 R, 18, 19; 328/229,

2,563,197 8/1951 Sziklai et a1. 313/75 X 2,609,500 9/1952 315/18 X 2,998,543 8/1961 Ross 315/27 R 3,195,008 7/1965 Charles 315/18 Primary Examiner-A1fred L. Brody Attorney, Agent, or Firm-Robert R. Tipton [5 7 ABSTRACT A beam of electrons is injected into a plasma confining magnetic field by directing the beam of particles along a trajectory which is generally parallel to the lines of force of the magnetic field with drift forces existing on the particles to cause them to drift across magnetic lines of force by either the use of an auxiliary magnetic field or by adjusting the parameters of the toroidal system when injecting into a toroidal confining magnetic field. The system is also adaptable to simultaneous injection of a plurality of beams and for injection times greater than one transit of the torus.

17 Claims, 11 Drawing Figures PATENTEUaunzomn FIG.7

mmmmmmmmmmmmmm.

PARTICLE BEAM INJECTION SYSTEM BACKGROUND OF THE INVENTION This invention relates generally to plasma confining apparatus and in particular to apparatus for injecting charged particles into a confining magnetic field.

It has been the practice for the prior art injection systems, when injecting charged particles into a plasma confining magnetic field, to inject the particles along a trajectory that is generally perpendicular to the lines of force of the magnetic field or, for a linear configuration of a confining magnetic field, to inject at one end of the system coincident with the longitudinal axis of the field.

Where injection is attempted perpendicular to the magnetic field, the beam is generally aimed so that it is off-center, that is, away from the longitudinal axis of the magnetic confining field, so that the charged particles enter the magnetic field and form a cylindrical orbit commonly referred to as an E-layer or P- layer depending upon whether the particles are electrons or protons, respectively. The use of such layers is primarily for the purpose of creating a strong magnetic field to ionize, compress, contain and/or heat the plasma.

Where injection is attempted along the longitudinal axis of the confining magnetic field, the magnetic mirror field at the injection end is reduced in magnitude during the time the particles are injected, after which it is returned to its original state to prevent the particles from escaping. The time period for such injection is, of course, limited by the round trip transit time of the particles along the length of the confining magnetic field.

In addition, in all prior art devices where a beam of charged particles is injected into a confining magnetic field, the trajectory of the beam particles were all generally parallel to each other.

SUMMARY OF THE INVENTION In the apparatus and system of the present invention, the beam of charged particles is injected into a plasma confining magnetic field with the axis of the beam of charged particles generally parallel to the lines of force of the magnetic field, and, in some situations, using a particle beam in which the particle trajectories have initial parallel and transverse velocity components. Injection occurs generally along the side of the containment vessel into the confining magnetic field. Adrift of the injected charged particles, which is controlled by various means, causes them to move across magnetic lines of force and to be confined in the magnetic field.

It is, therefore, an object of the present invention to provide a system for injecting charged particles into a confining magnetic field in which the trajectories of the charged particles are injected generally parallel to the magnetic lines of force of the field.

It is another object of the present invention to provide a system for injecting charged particles into a confining magnetic field in which the drift of the particles across the magnetic lines of force of the field is controlled.

It is a further object of the present invention to pro vide a system for injecting charged particles into a confining magnetic field in which the trajectories of the in- 2 dividual particles in the beam may be diverging initially with respect to the magnetic lines of force.

It is still another object of the present invention to provide a system for injecting charged particles .into a confining magnetic field in which the injector may be located within the confining magnetic field.

It is still a further object of the present invention to provide a system for injecting charged particles into a toroidal confining magnetic field in which injection time may be greater than the transit time of the particles around the torus.

It is yet another object of the present invention to provide a system for injecting charged particles into a toroidal confining magnetic field in which injection of charged particles can be made at a plurality of locations about the torus.

These and other objects of the present invention will be manifest upon careful study of the following detailed description when taken together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an isometric partial cut-away view of a typical toroidal confining magnetic field apparatus showing the typical injection system of the present invention.

FIGS. 2A, 2B, 2C, and 2D, are cross-section elevational views of the torus of FIG. 1 taken at intervals around the torus beginning at the point of injection of the charged particles.

FIG. 3 is a longitudinal sectional elevational view of a typical injector system of the present invention in which the injector assembly is located within the confining magnetic field.

FIG. 4 is a longitudinal sectional elevational view of a second type of injector system of the present invention in which the injector assembly is located within the confining magnetic field.

FIG. 5 is a longitudinal sectional elevational view of a typical injector system of the present invention in which the injector assembly is located outside the confining magnetic field.

FIG. 6 is a longitudinal sectional elevational view of a typical injector system of the present invention in which the beam of charged particles is initially divergmg.

FIG. 7 is a longitudinal sectional elevational view of a typical linear confining magnetic field apparatus showing the typical injector system of the present invention.

FIG. 8 is a schematic drawing of an initially diverging particle beam showing the trajectories of the particles as they enter the magnetic field.

DESCRIPTION or THE PREFERRED EMBODIMENT With reference to FIG. 1, there is illustrated an isometric, partial cut-away view of a typical toroidal plasma confining magnetic field apparatus 10 comprising, basically, a vacuum tight toroidal housing 11 having a major radius R and an axis of revolution 12, and surrounded by toroidal winding 14 which, when energized by power supply 15, creates a magnetic field within toroidal housing 11 for the containment, i.e., confinement of a plasma of charged particles 17 in the manner of a magnetic bottle.

As also illustrated in FIG. 3, projecting through the top of housing 11 is a typical charged particle injector assembly 19 comprising, basically, an anode conductor 21 coaxially' disposed about cathode conductor 22 and with a very thin electron transparent anode foil 23 disposed over one end of anode 21. Since, for most applications, the pressure within the space between anode 21 and cathode 22 is different from the pressure within housing 11, foil 23 is used to maintain the pressure difference and also act as an accelerating electrode.

There will be other applications, however, where it will not be necessary to maintain such a pressure difference and in such a situation other configurations of particle accelerators may be employed that use other types of accelerating electrodes.

The end of cathode 22 is spaced apart from anode foil 23 with anode 21 and cathode 22 connected to a high voltage power supply 24. When energized, electrons are caused to flow from the end of cathode 22 and be accelerated toward and pass through anode foil 23 to define a beam 25 of charged particles which will generally follow along the magnetic lines of force 26 of the confining magnetic field created by windings 14.

It will be noted in FIG. 5, as well as the remaining Figures, that the beam of charged particles 25, where it is shown as a dotted line as it enters the toroidal magnetic field, begins to follow a helical trajectory along the magnetic lines of force 26 following a beam path 25 as defined by arrows 28, with the predominant motion of the beam along the field lines.

In the case of toroidal apparatus 10, upon entering the magnetic field created by windings 14, the charged particles are subjected to various forces depending upon the various parameters of the particular system such as the geometry of the torus, the intensity of the confining magnetic field and the angle of divergence of the particle beam.

For example, it has been found that an appropriately charge and current neutralized electron beam will undergo a deflection or drift across magnetic lines of force, which drift is directly proportional to the distance the beam propagates around the torus and a function of the angle of initial divergence of the particle beam, and inversely proportional to the major radius of the torus and the magnitude of the toroidal magnetic field as follows:

d=KZ F()/R XB Equation 1 where:

d deflection or drift of the beam after a transit of a distance Z around the torus. Z distance the beam propagates around the torus.

R major radius of the torus.

B magnitude of the toroidal magnetic field.

F( 0) a function of the initial angle of divergence of the particle beam.

K a constant.

Thus the drift of the particle beam can be controlled by controlling the various parameters.

In order to follow beam 25 as it is deflected or drifts, FIGS. 2A, 2B, 2C, and 2D illustrate such dn'ft. FIGS. 2A through 2D are elevational sections taken at 90 degree intervals around toroidal apparatus beginning with FIG. 2A at the point of injection of the charged particles.

Beam 25, or more particularly, beam path 25, is shown in cross-section passing into the plane of the drawing after several transits around the torus. The first transit of the beam immediately after leaving injector assembly 19 is identified as beam 25, while the second, third, fourth, etc. transits are identified as beams 25', 25", 25", etc., respectively.

Unless counteracting forces are applied to beam 25, or the beam loses energy to the plasma, it will continue to drift across the magnetic lines of force until it reaches the other side of toroidal housing 11. If housing 11 is a conductor, beam 25 will reach an equilibrium position where the drift forces are balanced by the image or restoring forces created by housing 11. These image forces cause the beam not only to be repelled by the conducting wall of the torus, but they also cause the beam particles to rotate around the central axis of the toroid. When injector assembly 19 is located near the conducting surface of the torus, these image forces can thus be utilized as a further method of injection augmentation.

For Example, by placing a conductive liner or conductive deflection plate (not shown) proximate the injection region, at the regions highest current density, the image force rotation will be maximum and injection will be augmented.

A further use of this image or restoring force by housing 11 is made in the configuration shown in FIG. 4, in which injector assembly 19 projects through housing 11 into the toroidal magnetic field similar to FIG. 3, however, an electrically conductive deflector plate 27 is connected to housing 11 and tapers to meet and connect to the bottom of injector assembly 19 to define a ramp whereby beam 25 is deflected by an interaction of the toroidal field and the image or restoring force created by plate 27.

Deflector plate 27 is useful when the deflection of beam 25 is not sufficient to clear injector assembly 19 after one transit time around the torus or, where more than one injector assembly is used, the next injector assembly along the torus.

In certain instances, it may be desirable to initially increase the drift of the charged particles as they enter the toroidal magnetic field.

In such instances, the configuration shown in FIG. 5 may be used, which, in addition to toroidal housing 11 with magnetic field windings 14, comprises an injector housing 29 connected to and protruding above housing 11, and a magnetic field deflector or hernia coil 30 located immediately above the point of injection. of beam 25.

Hernia coil 30 is a solenoid coil which is used to pull, or distort, or perturb, the lines of force 26 of the main toroidal confining magnetic field upwardly so that they are generally tangent to the initial trajectory of the particles of beam 25.

By virtue of this distortion of the toroidal magnetic field, the rate of drift can be controlled and is caused to increase at the point of initial entry into the magnetic field thus pulling the particles more rapidly into the toroidal magnetic field while the rate of drift in the unperturbed field remains the same.

Injector assembly 32 of FIG. 5, in order to provide a beam 25 of high energy particles comprises, basically, a housing 33 coaxially disposed around cathode 34 and with an electron transparent anode foil 36 sealed over the end of housing 33 to maintain the pressure difference between the space inside housing 33 and the space inside housing 11.

Here again, where it is not necessary to maintain the pressure difference between housing 33 and housing 11, other types of particle accelerators may be used which do not employ an electron transparent anode foil.

As shown in FIG. 5, beam 25 comprises energetic electrons which generally follow along the lines of magnetic force but are caused to drift across the lines of force as previously described after entering the toroidal magnetic field.

It will be noted that FIGS. 1, 2, 3, 4, and 5 all illustrate the use of a beam 25 of charged particles in which the individual particle trajectories are generally parallel.

In FIG. 6, apparatus using a beam 38 (whose path to the left of reference numeral 38 is defined by some typical trajectories of the individual particles, and whose path to the right of reference numeral 38 is further defined by arrows 37) of charged particles is illustrated in which the individual trajectories of the particles is initially diverging. Such divergence is measured by the beam spread that would have occurred in the absence of the toroidal magnetic field and is related to the function F(l9) of Equations 1 and 2.

As can be seen more particularly in FIG. 8, an injector assembly 39 is shown in part with the initial beam spread is measured as angle 0 from the axis 44 of the beam, which axis is generally parallel to the magnetic lines of force 26 of the toroidal magnetic field.

The trajectories of the individual electrons is shown as dotted lines after they enter the magnetic field.

The apparatus of FIG. 6 comprises, basically, an injector assembly 39 having an anode 40 disposed coaxially about cathode 41 and arranged so that the electrons emitted from the end of cathode 41 are initially diverging. A thin electron transparent foil 43 is disposed over the end of anode 40 to maintain the pressure difference between that of injector assembly 39 and the space inside housing 11.

Again, other particle accelerator configurations may be used which do not employ an electron transparent foil where it is not necessary to maintain a pressure difference between the accelerator housing and the housing of the torus.

With respect to toroidal injection systems using a hernia coil, in some instances it may be desired to reduce the amount of drift or deflection of the beam of charged particles and, at the same time, keep certain of the toroidal system parameters unchanged. If this is desired, an auxiliary magnetic field may be used covering the entire torus, including the hernia coil, in which the magnetic lines of force are parallel to the axis of revolution 12 of the toroidal housing 11.

By the use of the hernia coil as shown and described for FIG. 5, the drift rate of the particles at the point of injection into the perturbed magnetic field is the highest and decreases to a slower but steadier rate in the unperturbed toroidal field. By providing an auxiliary magnetic field with lines of force parallel to axis 12 and of such a magnitude as to reduce the steady rate of drift in the unperturbed field to zero, particles can be continuously injected at the hernia coil region and maintained in the toroidal magnetic field without dependance upon image forces of the housing.

It can also be seen that, although only one injector assembly is shown in FIG. 1, a plurality of injectors can be used which are disposed about the top of housing ll. In such case, the deflection or drift of the beam must be sufficient to clear the next injector.

To operate the apparatus of FIG. 1, housing 11 is evacuated to a pressure of about 0.1 to 10 torr. Winding 14 is then energized to produce the plasma confining magnetic field within housing 11.

A plasma of charged particle such as protons and electrons can be introduced into the containment field by methods well known in the art, however, the plasma itself is not absolutely necessary to the operation of the injection system of the present invention, but, for the present illustration, can be a material that is to be heated by the use of the injection system of the present invention.

It is necessary, however, that the electron beam be charge and current neutralized. This can be achieved either through the use of (a) a neutral gas, (b) a neutral plasma, or (c) an electron beam that is too weak electrostatically and magnetically within itself to effect the operation.

For the pressure region investigated above, the function F(6) has been found to be approximately as follows:

F(l9) Cos 0 Sin 0/2 Cos 6 Equation 2 where 6 the angle of initial beam spread as measured from the axis of the beam if no magnetic field were present.

The pressure between anode 21 and cathode 22 is reduced to allow the formation of a beam 25 of electrons when anode 21 and cathode 22 are energized from high voltage power supply 24.

A satisfactory accelerating voltage has been found to be about 1,000,000 volts and is generally limited by the electrode spacing and insulation characteristics of the system.

Some typical parameters of the toroidal injection system of the present invention are as follows:

d 4 cm.

B 5,000 gauss R 60 cm.

Z 377 cm.

6 20 40 where d deflection of beam after a transit of a distance 2 around the torus.

B magnitude of magnetic containment field.

R major radius of torus.

Z distance propagated around the torus.

0 initial angle of divergence of the particle beam.

With reference to FIG. 7, there is shown a typical linear plasma confining magnetic field apparatus 45 comprising a containment vessel 46 surrounded by windings 47 for creating a linear plasma confining magnetic field illustrated by magnetic lines of force 48 with end coils 49a and 4% located at each end of the magnetic field in order to close off the ends and contain the plasma within vessel 46.

An injector assembly 51, similar to that shown in FIGS. 1 and 3, is disposed projecting into the plasma confining magnetic field in which the particle beam 52 trajectory is generally parallel to the magnetic lines of force 48.

For a linear configuration, it can be seen from Equation 1 that the value of R is infinite such that the drift, according to that Equation, would be zero. Therefore, a hernia or solenoid coil 54 is provided to cause an increase in the drift rate of the particles at the point of injection.

As previously described for the injection configuration of FIG. 5, using hernia coil 30, hernia coil 54 of P16. 7 performs the same function of distorting or perturbing the magnetic lines of force of the containment field to cause an initial increase in the rate of particle drift where the particles are injected parallel to the lines of force of the distorted field.

Once injected into the linear magnetic field by virtue of the drift forces, and where the particles then come under the influence of the undistorted magnetic containment field, the particles will stop drifting across magnetic lines of force and, since drift is zero in the linear configuration, they will be thus contained in the linear magnetic field.

We claim:

1. An apparatus for injecting charged particles into a confining magnetic field comprising means for producing a beam of charged particles,

means for creating a confining magnetic field for confinement of a plasma of said charged particles,

means for directing said beam of charged particles into said confining magnetic field along a trajectory generally parallel to the magnetic lines of force of said magnetic field, and

means for controlling the drift of said beam of charged particles to cause said particles to drift across the magnetic lines of force of said magnetic field.

2. The apparatus as claimed in claim 1 wherein said charged particles are energetic negative particles.

3. The apparatus as claimed in claim 1 wherein said charged particles are energetic positive particles.

41. The apparatus as claimed in claim 1 wherein said beam of charged particles is initially diverging.

5. The apparatus as claimed in claim 1 wherein said means for producing said beam of charged articles is disposed within said confining magnetic field.

6. The apparatus as claimed in claim 1 wherein said means for directing said beam of charged particles into said confining magnetic field further comprises means for deflecting said beam of charged particles away from said directing means.

7. The apparatus as claimed in claim 6 wherein said means for deflecting said beam of charged particles comprises means for producing an image force on said beam proximate the point of injection of said beam of charged particles.

8. The apparatus as claimed in claim 6 wherein said means for deflecting said beam of charged particles comprises I means for producing an image force on said beam after travelling around said magnetic field and before passing the point of injection.

9. The apparatus as claimed in claim 1 wherein said means for directing said beam of charged particles into said confining magnetic field further comprises means for perturbing the lines of force of said confining magnetic field at one location, and

means for directing said beam of charged particles tangent to said perturbed lines of force.

10. The apparatus as claimed in claim 1 wherein said means for directing said beam of charged particles into said confining magnetic field further comprises means for increasing the drift rate of said beam of charged particles at one location, and

means for directing said beam of charged particles into said magnetic field tangent to the lines of force of said field at said location.

11. The apparatus as claimed in claim 1 wherein said means for creating a confining magnetic field comprises means for creating a toroidal confining magnetic field, and

said means for controlling the drift of said beam of particles comprises the adjustment of the magnitude of said toroidal confining magnetic field and the major radius of said toroid to cause said beam of charged particles to drift across the magnetic lines of force of said confining magnetic field.

12. The apparatus as claimed in claim 11 further comprising means for increasing the drift of said beam particles at one location,

means for directing said beam of charged particles into said toroidal magnetic field tangent to the lines of force of said field at said location, and

means for neutralizing said drift of said beam particles in said toroidal magnetic field.

13. The apparatus as claimed in claim 11 further comprising means for perturbing the lines of force of said confining magnetic field at one location, and

means for directing said beam of charged particles tangent to said perturbed lines of force.

14. The apparatus as claimed in claim 13 further comprising means for creating a magnetic field whose lines of force are parallel to the axis of revolution of said toroidal confining magnetic field and of a magnitude sufficient to neutralize said drift of said charged particles across the unperturbed magnetic lines of force of said toroidal magnetic field.

15. The apparatus as claimed in claim 1 wherein said means for creating a confining magnetic field comprises means for creating a linear confining magnetic field,

and

said means for controlling the drift of said beam of charged particles comprises means for perturbing the lines of force of said confining magnetic field proximate the point of injection of said particles. 16. The apparatus as claimed in claim 15 wherein said means for controlling the drift of said beam comprises means for creating an auxiliary magnetic field perpendicular to the longitudinal axis of said linear confining magnetic field proximate the point of injection of said particles. 17. The apparatus as claimed in claim 1 wherein said means for directing said beam of charged particles is disposed along a side of said confining magnetic field. 

1. An apparatus for injecting charged particles into a confining magnetic field comprising means for producing a beam of charged particles, means for creating a confining magnetic field for confinement of a plasma of said charged particles, means for directing said beam of charged particles into said confining magnetic field along a trajectory generally parallel to the magnetic lines of force of said magnetic field, and means for controlling the drift of said beam of charged particles to cause said particles to drift across the magnetic lines of force of said magnetic field.
 2. The apparatus as claimed in claim 1 wherein said charged particles are energetic negative particles.
 3. The apparatus as claimed in claim 1 wherein said charged particles are energetic positive particles.
 4. The apparatus as claimed in claim 1 wherein said beam of charged particles is initially diverging.
 5. The apparatus as claimed in claim 1 wherein said means for producing said beam of charged articles is disposed within said confining magnetic field.
 6. The apparatus as claimed in claim 1 wherein said means for directing said beam of charged particles into said confining magnetic field further comprises means for deflecting said beam of charged particles away from said directing means.
 7. The apparatus as claimed in claim 6 wherein said means for deflecting said beam of charged particles comprises means for producing an image force on said beam proximate the point of injection of said beam of charged particles.
 8. The apparatus as claimed in claim 6 wherein said means for deflecting said beam of charged particles comprises means for producing an image force on said beam after travelling around said magnetic field and before passing the point of injection.
 9. The apparatus as claimed in claim 1 wherein said means for directing said beam of charged particles into said confining magnetic field further comprises means for perturbing the lines of force of said confining magnetic field at one location, and means for directing said beam of charged particles tangent to said perturbed lines of force.
 10. The apparatus as claimed in claim 1 wherein said means for directing said beam of charged particles into said conFining magnetic field further comprises means for increasing the drift rate of said beam of charged particles at one location, and means for directing said beam of charged particles into said magnetic field tangent to the lines of force of said field at said location.
 11. The apparatus as claimed in claim 1 wherein said means for creating a confining magnetic field comprises means for creating a toroidal confining magnetic field, and said means for controlling the drift of said beam of particles comprises the adjustment of the magnitude of said toroidal confining magnetic field and the major radius of said toroid to cause said beam of charged particles to drift across the magnetic lines of force of said confining magnetic field.
 12. The apparatus as claimed in claim 11 further comprising means for increasing the drift of said beam particles at one location, means for directing said beam of charged particles into said toroidal magnetic field tangent to the lines of force of said field at said location, and means for neutralizing said drift of said beam particles in said toroidal magnetic field.
 13. The apparatus as claimed in claim 11 further comprising means for perturbing the lines of force of said confining magnetic field at one location, and means for directing said beam of charged particles tangent to said perturbed lines of force.
 14. The apparatus as claimed in claim 13 further comprising means for creating a magnetic field whose lines of force are parallel to the axis of revolution of said toroidal confining magnetic field and of a magnitude sufficient to neutralize said drift of said charged particles across the unperturbed magnetic lines of force of said toroidal magnetic field.
 15. The apparatus as claimed in claim 1 wherein said means for creating a confining magnetic field comprises means for creating a linear confining magnetic field, and said means for controlling the drift of said beam of charged particles comprises means for perturbing the lines of force of said confining magnetic field proximate the point of injection of said particles.
 16. The apparatus as claimed in claim 15 wherein said means for controlling the drift of said beam comprises means for creating an auxiliary magnetic field perpendicular to the longitudinal axis of said linear confining magnetic field proximate the point of injection of said particles.
 17. The apparatus as claimed in claim 1 wherein said means for directing said beam of charged particles is disposed along a side of said confining magnetic field. 